One of the main areas of application for a magnet providing a region having a substantially homogeneous magnetic field with high flux density is in magnetic resonance imaging equipment. Such equipment when used for medical diagnosis provides information comparable to that provided by a computerized axial tomography scanner but without subjecting the patent undergoing examination to potentially harmful x-rays. Additionally, an MRI system offers other advantages over a computerized axial tomography scanner in that the operation of the former is not affected by the presence of bone. Ultimately the patient can safely remain in place long enough, and the MRI system can be designed to have sufficient resolution to allow observation of the body's take up of a drug.
MRI systems typically include a vacuum vessel containing a superconducting coil and a cryogenic system to maintain the coil below its critical temperature. The vacuum vessel defines a bore for receiving the subject of the imaging. Positioned inside the bore are gradient coils which are pulsed and function to determine spatial position in the imaging process. It is necessary that the signals generated by the gradient coils penetrate the first wall of the vacuum vessel (the cylindrical wall directly defining the bore and providing a vacuum/atmosphere interface).
An example of a vacuum vessel having a stainless steel first wall is shown in commonly-assigned allowed U.S. patent application Ser. No. 638,876, filed Aug. 8, 1984, the teachings of which are incorporaed herein by reference. This first wall is welded to stainless steel end plates. Eddy currents are generated by the signals from the gradient coils as they penetrate this wall which has a thickness of about 0.25 inch. The thicker the first wall, the longer the time required for penetration by the gradient fields. While this vessel performs satisfactorily, it is desirable to reduce the penetration time (make the first wall more "transparent" to the gradient field). Proposed fast imaging techniques include computer controlled pulse sequences which are increasingly complicated with sharper rise times approaching those of square waves.
Vacuum vessels are known which have fiberglass first walls. While such first walls are transparent to the gradient fields, they present reliability problems with respect to the vacuum sealing requirement because they cannot be welded to the end plates of the vacuum vessel and long term vacuum degradation due to diffusion may occur.